In the above mentioned method, a transmitter dipole and a receiver dipole, sensitive to an electromagnetic field are initially arranged in co-operation with the subsoil. An impulsive exciting current is then applied repeatedly to the transmitter dipole in order to define pulses of alternate polarity, each one being followed by zero current. The transient signal which is produced thereby is repeatedly detected at the output of the receiver dipole. A subsequent processing of the transient signals which are detected successively allows a figurative representation of the transient phenomenon to be obtained which facilitates interpretation by the geophysicist. In fact, it is only possible for the analysis to be effected by man in this method, as at present there is no theory or model which allows the direct exploitation of detected signals.
A characteristic of transient currents is that they have a very low amplitude, and are often disturbed by noises from various sources, notably noises from industry and from the ground. The presence of noise means that the geophysicist is often confronted with serious problems in interpretating the data and documents which he is given after processing.